This invention relates to a mixing apparatus for continuously mixing solid particles with liquids, and more specifically the apparatus is useful as a mixer wherein the sand or particles similar to sand are blended with a liquid and the resulting slurries are used for treating oil wells in petroleum recovery operations.
More specifically, the mixing apparatus of this invention useful, for example, in gas and oil recovery operations comprises a mixing chamber or circular housing and a slinger including a disc configuration with a plurality of vertical, upstanding blades or vanes mounted on the surface of the disc and a circumferential wall on the outer periphery thereof. The slinger is enclosed within the mixing chamber or circular housing and fastened to the end of a drive shaft rotated by a motor, e.g., an electric or hydraulic motor, located above the housing. A hopper, e.g., a funnel, is mounted above an inlet eye in the top of the housing for introducing solid particles, such as sand or the like, into the housing. Near the bottom of the circular housing is an inlet with a pump, e.g., a centrifugal pump, for pumping liquid into the housing, with the resulting liquid/solid mixture being discharged through an outlet located approximately opposite the inlet side of the housing.
During normal operations of the mixing apparatus, sand or other solid particles flow from the hopper in a continuous manner into the rotating slinger through the inlet eye at the center of the top of the housing. The slinger is rotated while the centrifugal pump passes liquids through the inlet and thoroughly mixes with the solid materials being thrown in an outward direction, i.e., in a centrifugal action from the slinger. The solid/liquid mixture is continuously discharged under regulated pressure through the outlet of the housing.
In general, oil and gas wells are fractured by introducing various compositions including solids and liquids into the well in a series of operations. One of these operations includes introducing a solid granular or particulate material which is thoroughly intermixed with a liquid prior to being pumped into the well formation. For example, in hydraulic fracturing, a mixture is pumped under pressure deep into the subsurface formation through a perforated well casing to fracture the surrounding rock. After the liquid portion comprising polymeric materials is withdrawn from the formation, the solid is left to prop open the fracture, thereby allowing gas or oil to flow through the fracture to the well bore and to the pipeline for distribution.
Heretofore, conventional blending apparatus required various processes for mixing large quantities of solids and liquids in order to maintain the composition in suspension when pumped over extended distances necessary to fracture the subsurface formations. For example, in U.S. Pat. No. 3,256,181, the liquid and particulate materials are intermixed by swirling the liquid, with the solids being introduced near the center of the rotation, and discharging the mixture through an impeller under sufficient velocity to cause intimate mixture of the materials. In U.S. Pat. No. 4,453,829, sand, for example, is blended with a gel to obtain a fluid mixture suitable for treatment of oil wells. The blender comprises a slinger with an impeller enclosed in a housing. The slinger and impeller are fastened together, with the impeller positioned beneath the slinger. The slinger has a toroidal configuration, and the impeller has a vortex configuration, with the slinger having a larger surface area than the impeller. The shape of the slinger and its size contribute to the pressure balance within the fluid composition as it circulates inside the housing during the mixing operation.
U.S. Pat. No. 3,256,181 discloses a mixing apparatus comprising an impeller disposed within a circular housing having a peripheral outlet and at least one peripheral inlet, in addition to an eye inlet opposite the longitudinal axis of the impeller. The solids and liquids are mixed by rotation of the impeller within the housing, while the fluid is supplied into the peripheral inlet at less pressure than that which is inherently generated within the mixer while solids are supplied through the eye of the impeller.
U.S. Pat. No. 3,423,075 discloses a mixing device for mixing various wet and dry materials which comprises a substantially cylindrical casing defining a mixing chamber into which the components are fed. The dry components are added to the mixing chamber through a funnel-shaped apparatus located at the top and center of the casing, while the wet component is fed through a plurality of feed pipes located at the top of the casing at the base of the funnel, thereby allowing the dry component to be always fed into the mixing chamber in the presence of the liquid. The mixing of the components is accomplished with a floating impeller characterized by a disc supported in a substantially horizontal plane in the mixing chamber on the lower extremity of a drive shaft projecting upward through the funnel. The upper surface of the disc is provided with a plurality of upstanding or vertical vanes, with the impeller being the only moving part of the mixing device which functions as a pump for positively forcing the mixture out of the casing to a discharge pipe, which is substantially vertically above the casing so as to provide hydrostatic pressure in the mixing chamber.
In comparison with the above-described prior art, the mixing apparatus of the present invention comprises a circular housing and a slinger including a flat bottom disc, a plurality of upstanding vertical vanes or blades with an outer cylindrical wall at the outer periphery of the disc. The blades may have the configuration of either a flat or a curved surface as they extend inward toward the center of the flat bottom disc. In the present mixing apparatus, the function of the ordinary impeller is replaced with an external pump, e.g., a centrifugal pump, controlled by variable speeds to maintain the discharge pressure of the solid/liquid mixture. By replacing the function of the impeller with the centrifugal pump, the mixing apparatus of this invention solves two major problems presently encountered in cement blenders.
More specifically, in most blenders, e.g., cement blenders, there is generally an inadequate eye diameter at the lower speeds of the impeller with a loss of prime at the higher rates of speed. In the instant mixing apparatus, however, the slinger with its outer wall creates a spinning open eye wherein the solids, e.g., proppants, are added to the swirling liquids in comparison to the closed system blenders, where the solids are incorporated by means of an impeller and an external suction pump; see U.S. Pat. Nos. 3,256,181 and 3,326,536.
In comparison, to increase the diameter of the eye for introducing solids, the instant apparatus uses an open slinger in the mixing chamber in combination with an external centrifugal or suction pump. The open slinger is rotated at speeds sufficiently high to hold back the desired discharge pressures. This is accomplished by using a slinger which comprises a flat disc with vertical vanes or blades and, more important, a cylindrical outer wall attached to the outer periphery of the flat disc. The flat disc with the vertical blades and the cylindrical outer wall creates an effective dead space near the outer wall. This structure makes the equipment less expensive in that the slinger will not wear out as frequently, since the solids, e.g., the proppants, fill the dead space created by the cylindrical wall, which provides protection from wear. The external pump is preferably a centrifugal pump with speed controls to maintain the discharge pressure of the mixer. Fluids from this pump flow into the mixing chamber of the housing tangentially or through nozzles which direct the stream and thereby achieve a swirling motion. By separating the slinger from the function of the impeller, the instant mixing apparatus essentially avoids two main problems. First is the inadequate diameter of the eye at low speeds, and the second is the loss of prime at the higher speeds. Moreover, it is possible also to increase the fluid flow rate capability of the mixer by increasing the size of the pump and/or the speed. Thus, not only is the instant mixer less costly to maintain because of the design, but it also has increased mixing capabilities.
There are many advantages in changing the design of the slinger, e.g., from a toroidal, concave, upward flow to a flat base disc with vertical or upstanding vanes with an outer wall which holds back the pressure to adequately mix the liquid with the solids. The flat disc slinger of this invention is less costly to maintain and more efficient to operate because of the cylindrical wall at the outer edge of the upstanding vanes where a dead space is created to which the solids, e.g., proppants, are added during the mixing operation.
Accordingly, one aspect of this invention is to provide a mixing apparatus capable of forming enlarged eye diameters for introducing solids in an open slinger in combination with an external pump.
Another aspect of this invention is a mixing apparatus comprising a slinger having a flat disc with a cylindrical outer wall capable of being rotated at speeds sufficient to hold back the discharge pressures which creates a dead space near the wall and prevents wear during the mixing operation.
It is an advantage of this invention to provide a mixing apparatus comprising a cylindrical housing and a slinger with an external pump which directs fluids tangentially into the housing to achieve a swirling action.
It is a still further advantage of this invention to provide a mixing apparatus comprising a cylindrical housing and a slinger wherein the impeller's function is replaced by a pump which adequately forms the eye diameter at low rates of speed and avoids the loss of prime at the higher rates of speed.
These and other characteristics of the invention will become more apparent from a further and more specific description of the invention as follows.